Existing force sensing devices based upon load cell technology usually utilize one of two designs. The first is a strain gauge adhering to an elastic spacer that produces changes in electrical resistance based upon a change in force or weight applied. The second is an elastic mat-type, wherein the elastomer is a sheet of plastic and acts as both a capacitor dielectric and as a deformable spring between the electrodes to which it adheres. This mat-type force sensing device typically measures a change in capacitance between the electrodes as force is applied to the device and the distance between the electrodes decreases accordingly.
Ideally, the relationship between force and the change in elastomeric material deformation is linear over a workable range of the elastic mat. Unfortunately, this relationship is often one of non-linearity because of the nonlinear characteristics of elastomeric material used and/or the elastomeric mat is in someway constrained during deformation. This is usually a result of elastomer form factor such as shape and density, space limitations for transverse elongation of the elastomer during deformation, and/or method and degree of securing the elastomeric spacer to the hardware.
Force measurements of mat-type force sensing devices also suffer from low sensitivity and low resolution because the smaller dielectric constant of the elastomer is used, rather than the larger dielectric constant of air. Sensitivity and resolution of the capacitance signal is also attenuated by the non-usable or non-deformable distance which separates the two electrodes of the variable capacitor.
Previous developers have used multiple elastomeric strips or multiple complex perforations through the elastomeric matrix to improve its direct linear response. These developers met with varying degrees of success and always an increase in manufacturing difficulty and cost.
As stated above, mat-type force or weight sensing devices were also limited in sensitivity and resolution by the limited usable travel distance of the elastomer as force was applied to deform the elastomer, and by the larger non-deformable distance that attenuates the measurement or capacitance signal. This deformable distance is only about twenty to thirty percent of the entire dimension of the elastomer, and is usually referred to as the linear travel distance. Any force or weight that deforms the elastomer beyond this twenty to thirty percent limit will cause the elastomer to physically breakdown resulting in an irreversible failure of the force sensing device.
Accordingly, there is a need in the marketplace for a force sensing device that has a physical structure which directly linearizes the relationship between applied force and measurement signal, and that provides greater sensitivity and higher resolution of those measurements. There is also demand for a device that is structurally designed so that physical breakdown of the elastomeric spacer upon an overload of force is mechanically prevented. Finally, there is a need for this device to be mechanically simple, easy to assemble and inexpensive to manufacture.